1. Technical Field of the Invention
The present invention relates to wireless (cellular) data communications networks and, in particular, to a method for reducing interference within a wireless data communications network.
2. Description of Related Art
Wireless data communications networks divide a large service area into a number of smaller discrete geographical areas called "cells" each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is at least contiguous and/or overlapping with multiple adjacent cells to provide substantially continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different assigned radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via communications links with a central control and routing station, commonly referred to as a mobile data intermediate system (MDIS), which functions to selectively connect data communications to the mobile stations through the base stations and, in general, control operation of the network.
Each cell is assigned use of a predetermined set of frequencies from the cellular frequency band for use in providing its control and data (traffic) channels. The assignment is typically made in accordance with a certain frequency plan. The frequencies used for the control channel and traffic channels assigned to a given cell are preferably spaced apart from each other across the frequency spectrum of the cellular frequency band. This serves to minimize the instances and adverse affects of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular frequency band, the same frequencies that are assigned to one cell are also assigned to (i.e., re-used by) other cells in distant parts of the service area. Typically, adjacent cells are not assigned to use the same frequency by the frequency plan. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength so as to limit propagation beyond the cell area. The foregoing precautions serve to reduce the instances of co-channel interference caused by re-use of that same frequency in a distant cell. It is further noted that careful power level control and distance assignment also assists in reducing the instances of adjacent channel interference.
In spite of the precautions taken by service providers in the frequency plan assignment for a frequency re-use wireless (cellular) data communications network and in the regulation of network operation, it is known that instances of co-channel interference do occur. This interference may be affected by a number of factors including: terrain irregularities; radio propagation changes; fading; multipath propagation; reflection; existence of human and natural obstructions; the number of available transceivers per cell; and variations in demand. This interference often adversely affects network operation by, for example, degrading quality on the traffic channels or interfering with the transmission and reception of control signals on the control channels.
One commonly employed frequency plan assignment for cellular communications networks comprises a seven-level plan wherein the cells covering the service area are grouped into clusters of seven cells each, and the available frequencies are divided amongst and re-used within each of the clusters. While providing satisfactory operation, interference concerns still exist with respect to the re-use of same frequencies in distant cells. Reduction in interference may be obtained by increasing the number of frequencies that are re-used in each cluster. However, such additional frequencies may not be available due to the limited nature of the allocated frequency spectrum for wireless cellular communications, and in some instances the addition of frequencies does not completely solve the interference problem. Accordingly, alternative mechanisms must be employed for reducing interference.
One solution is to partition each single frequency channel in the time domain to provide plural orthogonal logical channels which are used at different times (i.e., time division multiplexing). This solution advantageously does not require the allocation of additional frequencies from the wireless spectrum as the plural logical channels may share the same frequency. Time re-use is furthermore a particularly appealing solution because the level of mutual interference between the plural created logical channels is zero (i.e., the channels are orthogonal). Other solutions include spread spectrum code division and antenna sectoring (a form of space division) for providing additional channels within a frequency plan. Mutual interference with these solutions is not zero, but a sufficient level of interference suppression is provided to better support communications.
A known data communications network is the AT&T pACT '97 system whose design is based around the use of seven 12.5 kHz full duplex frequencies each supporting an eight kilobit per second (kbps) data rate. In this system, a 2.times. time and a 7.times. frequency re-use plan is implemented (i.e., two logical time division channels per frequency, and seven frequencies per cluster). Although a total of fourteen logical channels may be created with this plan, only a twelve level plan is implemented. The remaining two channels are held in reserve for use in specific geographical areas where interference is especially severe (such as hill tops). Each logical channel in the twelve level plan for the pACT '97 system supports an effective 4 kbps data rate.
In the pACT '97 system, a symmetric frequency assignment is provided with respect to the uplink and downlink. By this it is meant that the same number of frequencies are allocated for use in uplink communications as in downlink communications. Such symmetry is not, however, a requirement for operation of a wireless data communications network. For the narrowband personal communications service (NPCS), for example, the Federal Communications Commission (FCC) has defined an asymmetric frequency assignment comprising three 12.5 kHz frequencies on the downlink and one 12.5 kHz frequency on the uplink.
A need has arisen to extend the narrowband two-way paging and messaging protocol of the pACT '97 system to operate in the asymmetric frequency assignment environment of the NPCS system. The pACT '97 system design, however, is premised around the use of seven symmetric full duplex channel pairs (with twelve level re-use) which is not supported the NPCS asymmetric frequency assignment design. A direct application of the pACT '97 system design to the NPCS asymmetric frequency assignment design would require that the pACT '97 system twelve level re-use plan be applied to both the uplink and the downlink. As only one uplink channel is provided by the NPCS asymmetric frequency assignment design, and the pACT '97 system design requires full duplex operation, one proposed solution for supporting a twelve level re-use plan on the single frequency NPCS uplink is through a twelve level time division scheme. This proposed solution is unsatisfactory because the 8 kbps data rate would be reduced to a 0.667 kbps data rate per logical channel resulting in a large message latency. Is it further noted that the three downlink frequencies would require only a four level time division scheme to support the twelve level re-use plan. The proposed solution, which then would instead require use of twelve level time division on the downlink as well, is further unsatisfactory because downlink bandwidth (capacity) is wasted due to the constraints imposed by the limited uplink bandwidth (capacity).
There is a need then for a technique which would allow a lower level time division scheme to be implemented when a communications protocol designed for a symmetric frequency assignment design is utilized by a wireless data communications network having an asymmetric frequency assignment design. In particular, the need is for a technique to more efficiently allow the asymmetric frequency assignment design of the NPCS system to support the narrowband two-way paging and messaging protocol of the pACT '97 system. A technique which would allow for the use of a lower level time division scheme on the single frequency channel uplink would be ideal.